100 years at 6.7°C…

6.7°C / 44°F are such random numbers, and yet most buildings around the world are still designing their chilled water systems for their air-conditioning for these randomly specific numbers. Have you ever wondered why such a number is used? Why not use 6°C, or 7°C, or 10°C, or any other random temperature you can think of instead? Well there are two issues hidden within the answer to this question. 

Firstly, chilled water temperature set points were one of the trivial things that never got adjusted when the world started converting from Imperial to Metric. Essentially, since the dawn of commercial air conditioning about 100 years ago, systems were designed for 44°F, which is a nice whole number, but which converts to the infamous 6.7C. When countries around the world started adopting metric, for some reason chilled water temperatures got left behind. In some countries it has been adjusted in recent years to 7°C, which makes a little more sense, but that still raises the second issue. 

This second issue is perhaps the more significant issue that we should be questioning. Why is chilled water for commercial air-conditioning still being selected for 6.7°C/44°F, 7°C, 45°F/7.2°C etc? Studies have demonstrated that the most optimum room temperature is 22.5°C / 72.5°F to ensure everyone is comfortable in the space, however this is more than 15 degrees above the chilled water temperature.

Historically this stark difference in temperature made sense and would have been required for a few reasons. Firstly, the technology in the Air Handlers and Fan Coils was not all that advanced. Things like fin spacing, tube efficiencies, air speeds and volumes were rather imprecise sciences at the time, and as such, to deliver the required air temperature, you needed quite cold chilled water temperatures. Likewise, other technologies, such as chilled water pipe insulation would have resulted in some losses between the chiller and the coils, requiring the water to be colder coming out of the chiller. Humidity control in spaces was also not being measured or controlled accurately, and as such, colder air was viewed as superior to help dehumidify the buildings as well as cool them. However, it is now 2017, and all of these technologies, and measurements, and controls have come a very long way, and yet chillers are still being asked to deliver 6.7°C/44°F.

So the real question should now be, if we can deliver chilled water throughout the building with less losses, and air handlers and fan coils can deliver the required air temperatures with less of a temperature difference due to superior heat transfer, then why do we continue to use this low chilled water temperature.

Every decimal point increase in chilled water temperature would have a measurable improvement in the overall efficiency of the chiller. If given the opportunity, particularly with complete refurbishments of the system, or new construction, we should be analyzing this temperature requirement and looking at solutions at 8°C, 9°C, 10°C, 11°C, 12°C etc.

By Sam Ringwaldt